Quantum non-cloning theorem [1]} forbids perfect cloning of unknown quantum states or non-orthogonal states simultaneously. Based on the theorem, the first QKD protocol, BB84 [2]} has been proposed, in which non-orthogonal states are exploited to encode random key and distributed between two authorized users. Any probe from an unauthorized party intending to steal the key can be seen as a kind of clone, which will inevitably bring additional bit error according to the non-cloning theorem. This bit error can be found during the authorized users' post processing and thus the information theoretical security is achieved [3]}, [4]}, [5]}.
